This invention relates to sewing machines and, more particularly, to an arrangement for informing the sewing machine operator as to the amount of thread remaining on the bobbin of the sewing machine.
There are a number of known arrangements in the prior art for signalling a sewing machine operator as to the impending depletion of bobbin thread. Such an arrangement is desirable in order to warn an operator of impending bobbin exhaustion which might interfere with the appearance of a long seam. A number of these arrangements utilize a light source and a light detector arranged so that when there is thread on the bobbin the optical path from the light source to the light detector is blocked, this path being opened when the amount of thread remaining on the bobbin is depleted below some threshold value. Upon the occurrence of this latter condition, appropriate circuitry activates an alarm, or indicator, that warns the operator amount of thread remaining on the bobbin is below the predetermined threshold. Such an arrangement is disclosed, for example, in U.S. Pat. No. 4,188,901. Another arrangement of this general type is disclosed in U.S. Pat. No. 4,212,257 wherein an operator is advised both of impending depletion of the bobbin and also when the bobbin is full, the latter being for the purpose of preventing overwinding of the bobbin.
The prior low bobbin thread detection systems sense when the threaded bobbin diameter is below some threshold level. However, the bobbin thread is typically depleted from the bobbin in a non-uniform and unpredictable manner so that the threaded diameter of the bobbin is not uniform along the bobbin axis. Thus, there will be occassions wherein the optical path between the light source and the light detector is blocked by thread along one portion of the bobbin hub whereas another portion of the bobbin hub may have the thread completely depleted therefrom. Accordingly, the above-mentioned arrangements may not provide an entirely accurate indication of the amount of thread remaining on the bobbin. It is therefore an object of the present invention to provide a bobbin thread level detection and display system which is insensitive to variations in the threaded bobbin diameter along the axis of the bobbin.
The aforedescribed arrangements inform an operator when the amount of thread on the bobbin is above or below some critical value but do not provide any other information to the operator. It would be desirable to be able to inform the operator as to the level of thread on the bobbin even before the operator is advised as to the impending depletion of the thread. It is therefore another object of the this invention to provide an arrangement whereby the operator is continuously informed of the thread carrying condition of the bobbin over a range of such conditions.